Are inside of a firewall and need publicly-accessible name servers outside of your firewall.

Need name servers that are closer to the North American Internet backbones.

Providing additional secondary servers can greatly improve the reliability and accessibility of your domain. If your primary DNS becomes unavailable due to network problems, your secondary DNS entries come into play, so the more DNS servers you have, the greater the resilience to errors.

No-IP - Provides free dynamic and static DNS services under various domain names. With this free service you can run your own server and have one of their domain names (8 choices) point to your IP address.
You can also set the MX record for email. The dynamic DNS service also provides a downloadable client that keeps track of your changing IP address. They also provide free URL redirection using their domain hopto.org.